1. Field of the Invention
This invention relates to a granulating technique, and more particularly to a method of and an apparatus for manufacturing spherical particles used for pharmaceuticals, foods and the like; a method of coating powder onto these spherical particles as nuclei; and granulated products obtained by these methods and the apparatus.
2. Related Art Statement
Among the methods of manufacturing spherical or nearly spherical particles and of further making a powder such as a pharmaceutical powder adhere onto the particles, i.e. effectuating a so-called "powder coating", there are (1) a method in which powder is charged into a granulating vessel provided with a plane or dish-shaped rotary disk at the bottom thereof and a binder is sprayed to make the powder itself coheres, or further the powder is sprinkled thereover, and (2) a method in which particles as nuclei are charged into a granulating vessel, powder and a binder liquid are supplied thereto to make the powder adhere onto the surface of the nuclei, thereby obtaining granules of a desired shape.
In either one of the above-described granulating methods, in order to make granulated products of spherical or nearly spherical shape, or to make a uniform and dense layer of powder coating, it is necessary that the layer of the particles tumble centrifugally on the rotary disk of the granulating apparatus, and, to this end, the whole of the particles should not be in a floating fluidized state. In other words, in the granulating process with the above-described purpose, there should be the layer of the particles supported on the rotary disk.
The binder liquid for granulating and coating and the powder to be added during the operation are supplied onto these particles supported on the rotary disk. The binder liquid is usually added by spraying through a nozzle. In this case, unless the space density of the sprayed droplets is proper, the particles are locally excessively wet and agglomerations are generated, or wetting is so insufficient that the particles and the powder can not firmly adhere to each other or the powder itself can not firmly cohere.
The space density of the sprayed droplets decreases with increase of a distance between the supply port (spray nozzle) and the surface of the particle layer, so that it is necessary to make the distance within a predetermined range.
If the above-described distance is too small, the surface of the particle layer make a spray air stream turbulent, whereby the space density of the droplets is not uniform, and particles are excessively wetted in parts and insufficiently wetted in other parts.
On the contrary, if the above-described distance is too large, then the velocity of the sprayed droplets falls before they reach the particle layer, and, therefore, the droplets escape from the system because of the drying or fluidizing air, the droplets adhere to the wall of the apparatus to cause troubles, and/or a solvent is evaporated from the droplets to produce minute particles of the binder material (dusting phenomenon). Thus, there is a suitable range for the above-described distance not only from the requirements of the product quality but also from the process or operational requirements.
Furthermore, as regards supplying the powder, if the supply port for the powder is too close to the surface of the particle layer, then the density of the powder becomes excessively high locally, so that the uniform products cannot be obtained or the powder is agglomerated. On the contrary, if the supply port is too far from the surface of the particle layer, then the powder may adhere to the wall of the apparatus or the powder escapes from the system because of the drying or fluidizing air stream. From these reasons, there is a suitable range for the distance between the powder supply port and the surface of the particle layer also.
Now, in the granulation with the above-described purpose, quantity of the content in the granulating vessel increases gradually owing to the addition of the powder and the binder liquid, and as a result, the surface of the particle layer rises gradually with the progress of the granulation. Accordingly, the distance between the surface of the particle layer and the powder supply port or the spray nozzle decreases with time, so that the distance cannot be held within the suitable range during the whole process of the granulating.
However, there has not heretofore been known this fact that the distance between the supply port for the binder liquid to be sprayed or between the powder supply port and the surface of the particle layer constitutes an important factor for the finished quality of the granulated products.
Those skilled in the granulation work have experimentally known that, when a position of a spray gun is changed during the operation, a satisfactory result can be obtained in the finishing quality, yield and the like. It, however, has not been recognized that the above-described problem constitutes a main cause of the occurrence of troubles in granulating and coating processes, the problem being left unclarified, and the present inventors have clarified it for the first time.
As the simplest method to solve the above-described problem, an operator monitor and adjust the position of these supply ports in such a manner that the distance between the surface of the particle layer and the binder liquid supply port, or further the distance between the surface of the particle layer and the powder supply port are held within a proper range or, if possible, to a constant value at all times. However, the method like this is extremely troublesome, requiring much labor, lacking accuracy and being undesirable from the view point of Good Manufacturing Practice (GMP).